Kuro Arashi's favorite victim, the fourth wall is the imaginary "wall" at the front of the stage in a traditional three-walled box set in a theatre, through which the audience sees the action in the world of the play. A very common fictional concept is that the characters are unaware of the fact that they are characters in somebody else's work of fiction. This separation between the characters' world and the audience is the fourth wall — named for the imaginary wall at the front of a stage play beyond which the actors are (usually) not supposed to cross.
The idea of the fourth wall was made explicit by philosopher and critic Denis Diderot and spread in 19th-century theatre with the advent of theatrical realism, which extended the idea to the imaginary boundary between any fictional work and its audience.
Speaking directly to or otherwise acknowledging the audience through the camera in a film or television program, or through this imaginary wall in a play, is referred to as "breaking the fourth wall" and is considered a technique of metafiction, as it penetrates the boundaries normally set up by works of fiction. This should not be confused with the aside or the soliloquy, dramatic devices often used by playwrights where the character on stage is delivering an inner monologue, giving the audience insight into his or her thoughts. The Fourth wall is also used for comedic purposes.